F87 Boiler Fault Code: What It Means on Every Brand

The F87 fault code appears on 2 boiler brands — Glow-worm and Vaillant — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F87 means	Severity	DIY?
GWGlow-worm	The F87 code appears when your Glow-Worm boiler is unable to confirm that a flame is actually burning inside the combustion chamber. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.87	The F.87 code on a Vaillant boiler indicates a safety lockout triggered by incorrect flame detection — specifically, the boiler's control board is sensing the presence of a flame at a… Full guide →	Lockout — engineer likely	No

F87 brand by brand

Glow-worm F87

The F87 code appears when your Glow-Worm boiler is unable to confirm that a flame is actually burning inside the combustion chamber. The boiler relies on an ionisation (flame-sensing) electrode to detect the presence of a flame by measuring a tiny electrical current through it. When this signal is absent or too weak — whether because the electrode itself is faulty, its wiring is compromised, or the gas supply to the burner is unstable — the boiler locks out as a safety precaution to prevent unburned gas from accumulating. Glow-Worm shares its engineering platform with Vaillant, and F87 sits within a cluster of flame-detection faults (alongside F27 and F68) that all point to the boiler losing confidence in the burner flame. A single reset attempt is reasonable, but if the lockout returns immediately, a Gas Safe registered engineer is required.

Causes, fixes & costs for Glow-worm F87 → · All Glow-worm codes

Vaillant F.87

The F.87 code on a Vaillant boiler indicates a safety lockout triggered by incorrect flame detection — specifically, the boiler's control board is sensing the presence of a flame at a point when no flame should exist (i.e. when the gas valve is closed). Because this suggests a potentially dangerous situation, the boiler shuts itself down as a precaution. The fault broadly falls within the F.87–F.89 range of electrode and gas valve component faults. In practice, the root cause is almost always a worn or corroded ignition electrode, moisture-affected electronics, or in more serious cases a faulty PCB or gas valve regulation problem. The boiler will not restart on its own — a manual reset is required, and a Gas Safe engineer will typically be needed to diagnose and resolve the underlying issue.

Causes, fixes & costs for Vaillant F.87 → · All Vaillant codes

Frequently asked questions

Is the F87 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Vaillant the F87 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F87 fault myself?

Not usually. On every brand listed, F87 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F87 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.